It was recognized more than 20 years ago, that combining selective absorbers, vacuum insulation and nonimaging concentration (using Compound Parabolic Concentrator, or “CPC”, type optics as shown in FIG. 9A-9C) enabled stationary mid-temperature collectors to have a useful operating range approaching 300 degrees Celsius”. Following the early proof-of-concept experiments, a commercial collector was developed in the last 5-years with good performance up to 250 degrees Celsius. These configurations integrated all the optics within the vacuum envelope. For this reason we refer to them as ICPC's (integrated CPC's). Their cost of manufacture is presently too high for widespread applications. On the other hand, the advent of very low-cost evacuated tubes allows us now to consider these as candidates for low-cost mid-temperature applications. One can combine various of these features to use such low-cost tubes (intended as stand-alone low-temperature collectors for providing domestic hot water) as receivers and now combined with external nonimaging reflectors. Since these glass tubes were originally intended for low-temperature (domestic hot water) use, their use at higher temperatures raised issues such as providing for efficient heat transfer to a working fluid, and assuring against thermal-induced tube breakage.
A solar collector which is efficient at temperatures in the 125 to 150 degree Celsius above ambient range would therefore be of great utility for many high-value applications. For example, operating temperatures for solar cooling in conjunction with double-effect chillers are in this range. At the same time the collector component would need to be low-cost, have minimal operation and maintenance cost and long life. The external reflector form of a CPC has the potential for satisfying these criteria. The vacuum receiver has intrinsically long-life, being protected from the environment. The impressive commercial development of vacuum solar collectors in China over the last decade and more demonstrates that these can be manufactured and sold at low-cost. To give an example; in the year 2000 the all-glass dewar type solar tube made in China was available at an OEM cost of $3 US. Since the volume of manufacturing has been rising, prices are not increasing. It is significant to observe that a wide-angle CPC reflector will “unwrap” the cylindrical solar tube to an aperture of approximately 0.2 square meters. Therefore the vacuum component contributes $15 per square meter to the cost. The heat extraction device which may be a manifold likely adds a similar amount. The nonimaging reflector can be estimated at $20 per square meter, which is dominated by the material cost for a high quality aluminum mirror. An installed cost of approximately $100 per square meter would be a reasonable goal. The availability of an efficient mid-temperature solar collector for $100 per square meter would have a broad vista of applications.